This invention relates to a method of combining digital images in a digital color image processing system.
Digital color images can be digitally stored, processed, manipulated, altered and displayed on a digital color imaging system such as computer system including storage, processing capability, and a color CRT display.
A digital color image can be represented as a pixel map having a series of pixels, each pixel holding the color information for a single picture element. As used in this document, a pixel holds a color value relative to a large, abstract "color space" in which computations relating to color information can be performed. A pixel's color value may need to be indexed, altered, or otherwise transformed before it is suitable for use in "display space" for actual output to a display device. A pixel can have its color value represented in color components, such as a red component, green component, and blue component (RGB), which together define the color value held by the pixel.
A source and destination image can be combined or "composited" into a result image in which selected portions of the result image are from corresponding portions of the source image, and the remaining portions of the result image are from corresponding portions of the destination image. In the past, various selection processes have been used, such as methods in which source image replaces destination image in the result image when the source image value for a particular pixel is non-zero. Another method uses a bitmap as a mask, copying from the source where a bit in the bitmap is set (black), and copying from the destination where a bit in the bitmap is clear (white). This method of using a bitmap as a mask is documented in Inside Macintosh, Volume V, in Section 4, in particular pages V-70 and V-71, published by Addison-Wesley Publishing Company, Inc., Copyright 1986.
Compositing can also be used to "blend" two images by controlling the degree to which the two images are merged or averaged. This is often done by a method called "alpha channel blending" in which an 8 bit alpha channel controls the blending of two 32 bit RGB images.
However, while these methods have been useful, they are quite rigid and inflexible in accommodating images of various colors and bit depths, and not able to achieve certain desired effects, such as producing anti-aliased compositions with blended or "soft" edges, or in producing images with patterned or pictured text on a background image. What is desired is a compositing operation in which the mask can be a full color image, which accommodates various color and bit depths, can produce anti-aliased, blended edges, and can produce patterned or pictured text on a background image. The method of this invention provides these and other capabilities.